The objective of this project is to define the initial, intra-cellular events of steroid hormone action. Synthetic glucocorticoid derivatives, some of which react to form covalently labeled receptors via affinity labeling, have been prepared and are being used with glucocorticoid-responsive rat hepatoma tissue culture cells to examine: (1) steroid-receptor binding site interactions; (2) the effects of steroid binding on receptor conformation; and (3) the nature of "activation" of receptor-steroid complexes. With dexamethasone 21-mesylate, which we have patented as the first irreversible antiglucocorticoid, we have shown that both non-covalent agonist and covalent antagonist receptor-steroid complexes can be activated to apparently identical DNA-binding complexes. These results are incompatible with the current theories of antiglucocorticoid action. Investigations with two series of glucocorticoid derivatives (21-mesylate and 17-oxetan-3 feet-ones) indicate that the eventual expression of agonist vs. antagonist activity is determined by a balance of structural group determinants which are not restricted to a common region of the steroid.